what do you think...21mm or 24 mm?

Using a nex-5n I am anxious to know which focal lenght you would prefer in adition to my CV15 mm and for what reasons.

I'm not (at this point)interested in a special lens or brand only interested in terms of FOV.

With 15 mm I can surely crop to a fov similar to 21 or 24 mm but I will lose IQ, especially while the difference between 15 and 21 is quite big.

The gap between 21 and 24 is less and I think it is even lesser considering it is less super wide angle.

With a 24 mm lens I can't 'go back' while in the same situation I still have the option to crop with a 21 mm lens.(some times you are not able to step back to get he subject in frame e.g. a building where 15 mm is too wide)